Integrated optic devices with non-planar stripe waveguide structure have found wide applications in a variety of information handling systems because of their compact size and because their technology is compatible with that of the associated electronic circuitry.
One of the most noteworthy stripe waveguide structures presently used is the semiconductor diode laser which can advantageously be used for applications in data communication, optical memory and laser printing systems. In order to further improve performance, efforts are made to increase the scale of integration of opto-electronic integrated circuits. This requires replacing at least one cleaved mirror facet of the diode laser by an etched mirror. Good quality etched mirrors not only permit integrating monitor diodes with electronic circuits, but also facilitate such processes like mirror coating and testing at the wafer level. They also result in the added benefit of reduced handling, increased yield and decreased fabrication and testing costs. Etched mirrors make it possible to realize very short cavity lasers, groove-coupled cavity lasers, beam deflectors and surface emitters. In addition, new types of lower and waveguide structures with curved and angled mirror facets can also be fabricated using etching techniques.
A laser structure which has been determined to be a good choice for high performance applications is the so-called ridge GRINSCH (Graded-Index Separate Confinement Heterostructure) laser diode which is well known to the art. The article entitled "High-Power Ridge-Waveguide AlGaAs GRINSCH Laser Diode" by Ch. Harder, et al. (Electr. Lett., Vol. 22, No. 22, Sept. 25, 1986, p. 1081-1082 discusses such a structure in great detail. It provides high quality beams at extremely low power dissipation, is very efficient and has potential for high reliability, which is of utmost importance for high-speed optical interconnections. The ridge waveguide, which can be fabricated using simple and established processes, effectively stabilizes the transverse mode. GRINSCH laser diode structures with very low threshold currents and current densities have been reported.
Major requirements for etching laser mirrors include: high etch rates (4-6 .mu.m etch depth); low etch rate selectively of various materials of the laser structurae; vertical smooth facets with low damage; and surface roughness less than .lambda./10. Chemically assisted ion beam etching (CAIBE) is known as the best method so far for the fabrication of vertical etched mirror facets of III-V compound (e.g. AlGaAs/GaAs) laser structures with layers of varying Al concentrations. Such processes have been described in various publications, which are specifically incorporated by reference herein, namely, "Chemically Assisted Ion Beam Etching Process for High Quality Laser Mirrors" by P. Buchmann, et al. (Int. Conf. On Microlithography, Vienna, Sept. 1988), and "High Power Etched-Facet Laser" by P. Tihanyi et al. (Electr. Lett., Vol. 23, No. 15, July 16, 1987, pp. 772-773), which are incorporated by reference herein.
An additional requirement, planarity, also referred to as flatness of the mirror facets, is particularly important for single-mode lasers used in optical storage and in single-mode fiber communications. Any curvature of the mirror surface in the light mode region causes a phase-shift distortion in the reflected and the transmitted light. Reflected light with a strong phase front distortion has a low coupling factor to the back-travelling waveguide mode. Thus, the effective reflectivity of a curved mirror of a single-mode waveguide is educed and the threshhold current increases. Such phase distortions also shows up in the far-field of a laser output beam (side-lobes, multi-lobes). A non-Gaussian beam shape of the far-field indicates that the beam cannot be focussed to the ideal diffraction-limited spot using simple optics, resulting in a reduction of the possible storage density on, for example, a magneto-optic disk.
Planarity of the mirror facet is difficult to achieve when the laser structure has a pronounced topography as is the case for ridge waveguide and channeled substrate lasers, particularly where non-planar waveguide structures are used. Two effects are primary causes of the curvature of a mirror.
1. Topography/Lithography: Applying an etch mask layer causes some planarization, i.e., for a ridge laser, the mask is thinner on top of the ridge than on the etched horizontal surfaces on both sides of it. During mask fabrication, this causes an inward recess of the mask edge where the ridge is positioned. This recess is transferred to the mirror facet by an anisotropic etching process. Although this recess ranges from a few tens of nm to 500 nm, it is detrimental to the laser light wavefront (.lambda./2 in a GaAs laser corresponds to only 110 nm). In experiments, large wafer-to-wafer variations in the curvature of the mirror facets were observed, with the highest distortion occurring below the edges of the ridge.
2. Mirror etching process: The process used for the formation of the mirror facets (such as CAIBE) can also introduce a curvature of the surface depending on the anisotropy and the undercutting of the etching process. If chemically reactive gases are introduced to the etching system, the local concentration of active species will vary as a function of topography, resulting in an additional curvature of the etched facets.
This curvature problem has been recognized before and has been reported by N. Bouadma et al., in on article "GaAs/AlGaAs Ridge Waveguide Laser Monolithically Integrated with a Photodiode Using Ion Beam Etching" (Electr. Lett., Vol. 23, No. 16, July 1987, pp. 855-857) which is specifically incorporated by reference herein. The authors describe an attempt to solve the problem by shortening the ridge in the mirror facet region by a few microns. With this approach, topographic effects can be avoided but at the expense of wavefront and far field distortions which occur because the waveguide provided by the ridge is drastically affected.
Accordingly, it is a main object of the present invention to provide a method for improving the planarity (or flatness) of etched mirror facets of integrated optic structures with non-planar stripe waveguides.
It is another object of the invention to provide a method for etching semiconductor diode laser mirrors of the same or at least of similar quality than that of mirrors obtained by using conventional high-quality cleaving techniques.
Still another object is to provide a method for etching diode laser mirrors with extremely flat, smooth and vertical surfaces using simple, easy-to-control process steps.
A further object is to provide a diode laser structure having at leas tone etched mirror which is of the same or at least of similar quality than that of mirrors obtained by using conventional high-quality cleaving techniques.